Conventionally, a known high-pressure fuel pump may be directly equipped to an engine. For example, Patent Document 1 discloses a high-pressure fuel pump including a pump body and a plunger. The plunger is inserted in a cylinder of the pump body and is actuated by a cam, which is equipped to a shaft on the side of the engine, to move back and forth in the cylinder. The shaft on the side of the engine is, for example, a crankshaft, a valve camshaft, or the like. The fuel pump, which pressure-feeds high-pressure fuel, may be lubricated with oil to enhance robustness of a drive train. It is noted that, in a case where leakage of fuel and/or suction of oil occurs through a slidable gap between the cylinder and the plunger, lubricity of oil may be impaired, and/or fuel property may be degraded. Therefore, a high-pressure fuel pump having an oil-lubricating configuration may be equipped with a seal member to restrict leakage of fuel and/or suction of oil. In addition, a high-pressure fuel pump, which is configured to be directly equipped to an engine, may desirably have a configuration to avoid fallout of a plunger from a cylinder when the high-pressure fuel pump is mounted onto the engine.